Exclusive pumping should be able to help you keep up the supply as long as you do it regularly.
Yes, babies get angry when they’re hungry and your milk supply isn’t a lot yet, so topping up with formula may help to keep baby full and less angry. But if you’re planning to continue latching on, I would encourage you to keep letting baby latch and suck after you give some formula, because the sucking will help bring in the milk supply.
And yes, at this age, feed on demand. My baby sometimes feeds every 30min-1 hour when she’s cluster feeding. Don’t wait till 3 hrs.
Mummy J